It is known to adjust the brightness of an electric light by means of a dimmer. Dimmers supplied by AC power commonly use a triac or similar semiconductor device, which limits delivered current by switching on for a portion of each half cycle of the AC supply.
In residential applications, a dimmer for lighting is often in the same unit as an on-off switch for the lighting. Examples of such combined switches and dimmers are the “Skylark” S 600 and S 600P dimmers sold by Lutron Electronics Co., Inc., of Coopersburg, Pa.. The Skylark S 600 dimmer has a dimming control slider that switches the power off at the low-power end of its travel. The Skylark S 600P dimmer has a dimmer slider and a separate on-off switch. Both of the Skylark dimmer switches are designed to fit within a standard single-gang switch wall-box. They can thus be directly substituted for a standard light-switch.
It is also known in residential installations for a lighting wall-switch to control one or more electrical receptacles, so that floor or table lamps can be plugged into the receptacles and controlled by the wall-switch. Switch controlled receptacles are usually standard 110-volt receptacles adapted to receive standard 110-volt plugs. Such a construction provides for alternative control of a lamp plugged into a 110-volt power receptacle from either a wall-mounted switch or a switch provided by the lamp itself.
Control of a standard receptacle by a lighting dimmer, however, could have undesirable results. In the United States, the standard electrical receptacle used in residential applications is nominally capable of supplying 15 amps at 110 volts, which is equivalent to 1.65 kilowatts. A typical lighting dimmer is not designed to handle such power. For example, the above-mentioned Skylark models of Lutron Electronics Co., Inc. are rated for a maximum load of 600 watts. If a power load connected to a dimmer were to draw more than the rated capacity for the dimmer, the dimmer could be damaged.
Moreover, restricted electrical voltage supplied to a motor load connected to a dimmer-controlled receptacle could lead to motor stalling. Some motors designed for a 110-volt power supply, for example, may stall if the voltage drops below even 100 volts. An electric motor that remains stalled for a long period of time with even a small amount of current flowing through it could overheat, potentially starting a fire.
Receptacle units having two receptacles within a common housing are known. It is also known to install these double receptacle units such that a wall-mounted switch controls only one of the receptacles. Each of the receptacles, however, typically include identical openings for receipt of a standard plug. The receptacles, therefore, provide no indication to a potential user that given receptacle is controlled by a switch. Consequently, it would be easy for a user to inadvertently plug a 15 amp motor load, or other load that might overload the dimmer or might be damaged by the reduced (dimmed) voltage, into a receptacle controlled by a switch.
Dimmer control of a wall-mounted receptacle would desirably provide for control of a dimmable lamp plugged into the receptacle. In commercial and large residential applications, a lighting dimmer is often located in a remote location, such as an electrical closet or basement. These dimmers often control lighting in large spaces, which can include permanently installed lamps. Even in such settings, however, the ability to dim lamps plugged into wall-mounted receptacles might be desired.
Although desirable, the above-described consequences associated with dimmer control of a standard receptacle render such an installation inadvisable. The 2002 edition of the U.S. National Electrical Code, Article 404.14(E), permits dimmers to be used only where the lights are permanently wired to the dimmer, and not where they are plugged into a receptacle.